By using statistical models, the researchers found that for every extra year of education women had, the death rate for children under five dropped by almost 10 percent. In 2009, they estimated that 4.2 million fewer children died because women of childbearing age in developing countries were more educated.

In 1970, women aged 18 to 44 in developing countries went to school for about two years. That rose to about seven years in 2009.

The study was paid for by the Bill & Melinda Gates Foundation and was published Friday in the British medical journal Lancet.

"Investments in education pay off (by providing) better health in the future," said Emmanuela Gakidou, an associate professor at the Institute of Health Metrics and Evaluation at the University of Washington and the study's lead author.

Educated women tend to use health services more and often make better choices on hygiene, nutrition and parenting.

"This reminds us that in addition to having crucial interventions like immunization, we need to invest more into education," said Dr. Mickey Chopra, the health chief at UNICEF, who was not involved in the research. Chopra said more money should be invested in education but not at the expense of health programs.

Gakidou said considerable progress was made in Asia and Latin America, where women in some countries are more educated than men. But she noted a dismal situation in six countries where women typically go to school for less than a year: Afghanistan, Burkina Faso, Chad, Mali, Niger and Yemen.

Still, not everyone was convinced that the study's conclusions were right.

"It sounds plausible that education is related to child mortality, but finding a correlation does not prove causation," said William Easterly, a professor of economics at New York University who specializes in foreign aid.

He questioned the statistical methods used in the paper and said the authors had not adequately considered other factors that might have been responsible for the fall in child deaths.

Others said the focus should be on economic development rather than on specific health or education initiatives.

"Education is not much good if the health facilities and infrastructure don't exist," said Philip Stevens, a senior fellow at International Policy Network, a London-based think tank. "If a country is massively misgoverned, like Sierra Leone, no amount of education is going to put bread on the table for children."